AI in dota2 vs AI in starcraft
I can't answer definitively without a detailed breakdown of game mechanics of dota2 vs. Starcraft, but assuming the games have similar complexity, the AI controlling multiple in-game agents that form a team would be more complex, and therefore more difficult to create, than a single agent "team".  

My supposition is based on the fundamentals of combinatorics and computational complexity:


in dota2, the AI has to not only to identify winning strategies, but coordinate multiple agents.  


by contrast:


the Starcraft AI only has to identify a winning strategy, with no coordination between discrete agents which form a team.




Coalitions have traditionally been difficult to analyze in Game Theory and Combinatorial Game Theory (in the latter case especially, restricting mathematical study, for the most part, to games involving only 2 players.)  In this case the issue is not coalitions--the multiple game agents are already on the same team--but complexity is nevertheless dramatically increased by the existence of multiple agents on a team as compared to a team consisting of a single agent.